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Physics > Module 5 Thermal Physics > Flashcards

Module 5 Thermal Physics Flashcards

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1
Q

Define the absolute scale of temperature.

A

The scale of temperature does not depend on any physical property of any substance/matter and is measured in Kelvin.

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2
Q

What is temperature?

A

It is a measure of kinetic energy of particles in a body.

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3
Q

What do scales other than the absolute scale of temperature depend on?

A

They depend on a physical property of a specific substance.

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4
Q

What is 0 Kelvin equal to in Celsius?

Show equations

A

T=C +273

0-273= C

C=-273

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5
Q

Describe absolute zero.

A

It is the lowest possible temperature where internal energy of matter is minimum.

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6
Q

Define thermal equilibrium.

A

Two or more bodies in contact, with the same temperature and no net flow of heat are in thermal equilibrium.

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7
Q

Define a solid in terms of spacing, ordering and motion of particles and attractive forces.
What are two physical properties solids have?

A

Solids have particles packed together in a lattice, with the particles vibrating about their fixed positions. The forces of attraction between particles are strong.

Density is high
They have a definite shape.

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8
Q

Define liquids in terms of spacing, ordering, motion of particles and attractive forces.

A

Liquids have weaker forces of attraction between particles, compared to solids and hence have particles sliding past each other. Mean separation of particles is greater than solids.

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9
Q

Define gasses in terms of spacing, ordering, motion of particles and attractive forces.

A

Gasses have the lowest forces of attraction between particles and hence have a very high mean separation between particles. The particles roam about randomly in a gas at high speeds.

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10
Q

How different are densities of solid, liquids and gasses?

A

Densities are slightly different between solids and liquids but gasses have densities smaller by order of thousands

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11
Q

Do densities tend to change for liquids and solids in certain temperatures and pressure?

A

Not for liquids but densities of gasses can change.

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12
Q

Give equation for density other than D=M/V.

A

D= mass of one molecule in kg x number of molecules per cubic metre

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13
Q

Define change of phase.

A

A change of phase may be due to thermal energy entering or leaving a system. Particles may gain internal energy and increase their mean separation until they have enough energy to overcome attractive forces or vice versa

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14
Q

Define internal energy.

A

The sum of all kinetic and potential energies of a system

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15
Q

Where is potential energy stored?

A

It is stored within the bonds and intermolecular forces between molecules.

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16
Q

What happens to potential energies when substance is melting or evaporating?

A

They are released.

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17
Q

Describe the forces acting between molecules as they move from their equilibrium separation.

A

If two molecules move towards each other the resultant forces will be repulsive and be positive if they move further apart from one another.

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18
Q

Describe increase in internal energy as a body heats up.

A

The kinetic energy increases as their vibrations increase in amplitude, this in turn increases the potential energies too until the separation of particles is too high for attractive intermolecular forces to keep them together.

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19
Q

How big is the potential energy in gasses and describe the Maxwell Boltzmann curve .

A

The potential energy is very low.

The curve’s peak shows the most probable speed of a gas particle at given conditions of temperature, pressure.
The peak shifts towards the right when temperature is increased but the area under the graph remains the same.

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20
Q

Describe changes in internal energy during phase change.

A

Kinetic energy remains the same, hence temperature remains the same but potential energy is increasing or decreasing, overcoming intermolecular forces or allowing intermolecular forces to bind molecules.

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21
Q

How can you increase internal energy of a gas without heating it?

A

Compressing it, as this causes indirect heating

22
Q

Define brownian motion in terms of kinetic model of matter.

A

It is the random motion of visible particles in air caused by the collisions between many smaller air particles.
At non zero kelvin temperatures all matter have some kinetic energy.

23
Q

What kinetic energies are observed in solids, liquids and gasses?

A

In solids all ke is vibrational.
In liquids there is some translational ke but mostly vibrational.
In gasses all ke is translational

24
Q

How can you observe Brownian motion?

A

Fill smoke particles in a glass tube by igniting a straw. Cover the tube using a cover slip. Focus a light source towards the tube and view the smoke particles under a microscope.

25
Q

Does the smell of perfume spread because of brownian motion?

A

No, they are mostly spread due to convection.

26
Q

Define specific heat capacity, give units and relate it to change in energy.

A

It is the energy required to change the temperature of 1kg mass by 1 Kelvin.

c=J/kg K

E=mc DeltaTheta

27
Q

Why can’t you use E=mc Delta theta, for a substance changing phase.

A

During change of phase there is no change in kinetic energy and hence no change in temperature. So this formulae can’t be applied as E is directly proportional to Delta Theta

28
Q

Define a method to calculate the specific heat capacity of a material experimentally.

A

Find mass ‘m’ of the material, using a weighing scale. Insulate the material using an insulating foam. Insert an immersion heater inside the metal, also insert a thermometer. Dip the immersion heater and the the tip of the thermometer in some vaseline, to ensure contact between the apparatus. Connect the heater in series with an ammeter and connect a voltmeter in parallel. Turn the heater on and using a stopwatch measure the time elapsed to increase the temperature by at least 20 degrees.

Using E=Pt = IVt

IVt=mcDeltaTheta find ‘c’.

29
Q

Define specific latent heat.
Give types and meaning of specific latent heat.
State the sign used to represent Specific latent heat and it’s S.I unit.

A

It is the energy required to change the phase of 1kg of a particular material.

Latent heat of fusion, the energy required to melt or freeze a material.

Latent heat of evaporation, the energy required to boil a material.

L=J/kg

30
Q

Define an experiment to find the specific latent heat of fusion of water.

A

Take two funnels and fill them with ice, put an immersion heater in both but only connect one with a power supply and an ammeter in series. Attach a voltmeter in parallel to this heater. Start the heater once both the funnels start dripping water into beakers placed beneath them. And measure the time elapsed using a stopwatch
Once a significant amount of water is collected in the beaker with the immersion heater on. Find the difference in the masses of the water collected in both(this gives mass of ice melted due to heater)

Use E=mL
IVt=mL
L=IVt/m

31
Q

How can you experimentally find specific latent heat of vaporisation for water?

A

Take an insulated beaker full of water (could use polystyrene cup)and insert an immersion heater, and measure the mass of the beaker using a weighing scale. Connect the heater to a power source, an ammeter in series and a voltmeter in parallel.
Measure the time elapsed using a stopwatch and allow the heater to evaporate a significant amount of water. Stop the heater and measure the mass again and find the difference in mass.
E=mL
L=E/m
L=IVt/m

32
Q

Define 1 mol, how many particles in one mol?

A

It is the number of particles 12 grams of Carbon 12 atoms contains.

It equals 6.02x10^23 particles

33
Q

How many grams of oxygen gas form a mol?

A

Mr of O2= 32

Hence 32 grams.

34
Q

What are the assumptions made for kinetic theory of gasses?

A

1, Random motion of large number of particles
2, There are no Attractive forces between gas particles, except during collisions.
3, The Volume of gas particles is negligible, compared to volume of container,
4, Only Elastic collisions take place.
5, Duration of collisions is negligible compared to duration bw collisions.

35
Q

How can you make real gasses behave like ideal gasses and what are their differences?

A

Keep them at a temperature much higher than their boiling point (This will ensure high ke) and a low pressure.(This will ensure that particles are far apart and hence have low pe)

Normal gasses have internal energy in the form of ke and pe, whereas ideal gasses only have ke and no pe.

36
Q

What is change of momentum in a perfectly elastic collision?

A

2mv

37
Q

Derive pressure=m^2/v

A

Since the change in p = 2mv
The distance travelled before another collision with a particular wall=2L

The time taken for collisions between particle and wall= 2L/v

Since the force applied by the wall is = change in p over time, by Newton’s second law.
F=2mv/2L/v, F=mv^2/L

Since the molecule exerts an equal and opposite force on the wall, by newtons 3rd law.
The area of wall = L^2 and Pressure=F/A
Pressure = mv^2/L/L^2 = mv^2/L^3 or mv^2/v.

38
Q

Define pressure in terms of kinetic model.

A

Since all particles are in random motion , the collisions between particles and container wall bring about a force exerted on the wall due to change of momentum when particle hits. The mean force of all the particles hitting divided by the area of container gives pressure.

39
Q

Give equation for pressure in terms of Number of particles and r.m.s

A 
PV=1/3 Nm x rms, 
Where p is pressure,
V is volume of container,
N is total number of particles,
m is mass of one particle and
rms is root mean square speed.
40
Q

What is rms?

A

It is the square root of the mean speed of gas particles.

41
Q

Describe an investigation used to demonstrate Boyle’s Law.

A

Take an apparatus with a measuring tube closed at one end, with some air above oil filled at the other end. The oil is connected to a pressure gauge and a pump. Using the pump build pressure in the apparatus and allow the volume of the gas to decrease. Allow the gas to reach room temperature for some time and then take measurements for volume and pressure. Then decrease pressure in the tube and again allow some time to pass to allow gas to reach room temperature, take measurements of volume and pressure. Keeping decreasing pressure till at least six readings have been obtained.

Plot a graph of P against 1/V, the graph should be straight line through the origin.

42
Q

What things must you consider while doing Boyle’s law experiments to make sure results are accurate.

A

Allow heat to leave gas while it is compressed and allow gas to warm up when is it expanded.

Wear eye protection and constantly check for leaks of oil or pressure.

43
Q

Define Boyle’s Law.

A

The pressure of a gas in a system is inversely proportional to the volume, granted that Temperature is constant.

44
Q

Define the pressure law.

A

Pressure is directly proportional to absolute temperature for a constant volume.

45
Q

Define an experiment to demonstrate pressure law.

A

In a flask fill a fixed known mass of gas and connect the flask to a pressure gauge. Put the flask in a beaker of water with a bunsen burner beneath it. Insert a thermometer in the water and measure the initial temperature and pressure. Ignite the flame and measure changes in pressure for every 1 degree increase.

Plot a graph of P against T, this would be a straight line touching the x axis at -273 Celsius, when extrapolated.

46
Q

Define Charles’ law.

A

The volume of a gas in a system is directly proportional to temperature for fixed pressure.

47
Q

Give equation of ideal gas.

And define relation between Nk and nR

A

PV=nRT or PV=NkT

Where ‘k’ is Boltzmann constant and ‘R’ is Gas constant and is equal to k x Avagadro’s constant = 8.31.

Hence Nk=nR

Where ‘N’ is total number of particles and ‘n’ is number of moles of gas

48
Q

Is it okay to use any scale of temperature for calculations of ideal gasses?

A

No, the absolute scale must ‘always’ be used.

49
Q

What os relation between k and R

A

k= R/ 6.02x10^23

50
Q

Derive the equation Ek=3/2 kT

A

PV=1/3 x Nm x rms
PV=NkT
1/3 x Nm x rms = NkT

1/3 x m x rms =kT

Ek= 1/2 x m x rms
rms=2Ek/m

1/3 x m x 2Ek/m = kT

2/3 Ek = kT

Ek = 3/2kT

Physics (16 decks)

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